Cold seal packages have openings that are sealed under the application of pressure without the need for the application of elevated temperatures. Cold seal packaging can be used to package a variety of goods, including comestibles, pharmaceuticals, and medical supplies. They typically utilize cold seal adhesives such as natural rubber (also referred to as latex rubber). Such cold seal adhesives are differentiated from heat seal adhesives in that heat seal adhesives typically require both elevated temperature and pressure for activation. Cold seal adhesives are used as an alternative in packaging because heat seal adhesives have limitations in manufacturing. For example, heat seal adhesives require additional time for thermal diffusion to activate them.
Natural rubber (or latex rubber) has several disadvantages. Of particular concern is that it can initiate an allergic response in people. It is believed that certain people become sensitized to allergens in natural rubber over repeated exposure to natural rubber and, thus, exhibit increasingly severe allergic responses upon each exposure. This is particularly significant in the medical area where both health care workers and chronically ill patients are repeatedly and directly exposed to products, such as natural rubber gloves, tubing, and the like. To a lesser extent, medical packaging for wound dressings and bandages may also contain natural rubber in the cold seal adhesives used in packaging. Other disadvantages of natural rubber include discoloration upon aging and an unpleasant odor.
Synthetic cold seal adhesives have been incorporated into packaging to overcome the disadvantages associated with natural rubber. For example, U.S. Pat. No. 4,442,259 (Isgur et al.) describes the use of aqueous based polyurethanes in cold seal packaging applications. U.S. Pat. No. 5,616,400 (Zhang) describes a cold seal adhesive for use in forming packages. The cold seal adhesive is formed from a polyurethane ionomer, wherein overlapping coatings of the cold seal adhesive are pressed together forming an envelope enclosing an item packaged. That is, the seal is formed between the same substrates and same adhesives with no difference in adhesion at the two interfaces between the layer of adhesive and the substrates.
While these patents have described the adhesive characteristics of these synthetic cold seal materials, the opening characteristics of packaging remain an important consideration. This is of particular concern in packaging pharmaceutical and medical supplies to maintain sterility within the package. Evidence of tampering or breach of the cold seal are important features of such packaging. However, a competing interest is also opening the bond formed in a package with a cold seal material. For example, it is desirable that the package be easily opened with controlled predictable motion and force resulting in a decreased likelihood of spillage of the package contents.
Thus, what is yet needed is a cold seal package which exhibits sufficient bond strength and yet is easily opened and preferably provides evidence of prior opening.